In 1977, the first case of Percutaneous Coronary Intervention (PCI) surgery in the world was accomplished in Switzerland. In this surgery, a balloon was employed to dilate the position of the left anterior descending branch stenosis of the patient, successfully. However, the re-stenosis rate of the blood vessel after balloon dilation was very high, over 50%. The main reason was that, after the blood vessel was dilated and the external force was removed, the blood vessel may elastically recoil. In the 1980s, blood vessel stents widely used for coronary artery were invented. There were blood vessel metal stents mainly made of stainless steel, cobalt-chromium alloy or nickel-titanium alloy. After implanted into the human body, the stents may provide durable mechanical support for the blood vessel, thereby preventing the blood vessel from elastically recoiling and reducing the re-stenosis rate of the diseased blood vessel. However, since the stents were permanently implanted into the human body, the stents as foreign substances may cause intimal hyperplasia and quite high re-stenosis rate of the blood vessel. Since 2003, the application of the drug stents reduces the blood vessel re-stenosis rate to about 10%. However, since the drug stents still existed inside the human body permanently, the stents as foreign matter have great differences with vascular tissues in terms of mechanical property, which may cause chronic injury of the blood vessel, cause blood vessel medial atrophy and intimal hyperplasia at the late stage, and may also cause blood vessel re-stenosis finally, thereby limiting the further reduction of the blood vessel re-stenosis rate. As for children, the implantation of blood vessel stents with fixed sizes may hinder the gradual increase of the blood vessel, being unable to meet the requirement on the growth of children.
Both of the above intervention treatment technologies for blood vessel stenosis have defects: for dilating the blood vessel by using balloons, although the short-term effect is good, the re-stenosis rate at the late stage is very high because of elastic recoiling of the blood vessel; for implanting metal stents into the blood vessel, the metal stents including bare stent and drug stent may dilate the narrow blood vessel and provide a durable mechanical support, however, the blood vessel intima may be injured during the dilation of the metal stents, which may induce blood vessel intimal hyperplasia to lead to re-stenosis. The metal stents further have the defects of thrombosis, coagulation complication, flexibility mismatching, and increase of the re-stenosis occurrence rate at late stage if permanently remaining inside the human body.
In order to solve the above problems, in recent years, many people have begun to pay close attention to blood vessel stents that are absorbable by the human body. After a blood vessel stent is implanted into the human body, the ideal situation should be that, at the initial stage, the blood vessel stent provides a sufficient support to the blood vessel, and meanwhile releases drugs to treat the diseased blood vessel, and after the treating function is accomplished, the blood vessel stent is gradually absorbed so as to prevent the re-stenosis. According to different materials, there are two main kinds of absorbable blood vessel stents, one kind is the blood vessel stent made of macromolecular polymer materials, such as polylactic acid; and the other kind is the blood vessel stent made of metal materials, such as magnesium alloy and iron.
Recently, many people have been doing research on making blood vessel stents, for example, a polylactic acid stent, by degradable macromolecular materials, and some research results have started clinical tests. However, compared with the metal materials, the degradable macromolecular materials have obvious defects in terms of mechanical property, and so the application thereof is limited. When compared with the metal materials, the macromolecular polymer materials have low mechanical property and insufficient strength. In order to achieve certain radial support, the wall thickness of the stent must be increased. However, after implantation inside the human body, the stent with a thick wall may hinder the flow of the blood. In addition, the retraction rate of the macromolecular polymer material stents is quite high after dilation by the balloons, and so the expansion ratio of the diameter of the stent after dilated with respect to the diameter of the blood vessel is larger than that of the metal material stents, which may cause greater injury to the blood vessel during the dilation. The X-ray visibility of the macromolecular polymer material stents inside the human body is poor, and so it is difficult to locate and observe the stents during the implantation.
At present, there are mainly two types of metal materials applied to absorbable blood vessel stents: magnesium alloy material and pure iron material. The magnesium alloy material is poor in mechanical property and corrodes quickly. The magnesium alloy material has good bio-compatibility. However, the maximum elongation rate of the magnesium alloy stents is low, which brings great challenge to the structural design of the magnesium alloy stents, and so it is difficult to ensure the good mechanical property of the stents. Furthermore, the corrosion rate of the magnesium alloy stents is too fast, so a complex material manufacture process must be adopted to control the rate by which the magnesium alloy stents are absorbed by the human body. Iron is an essential element for the human body, and simultaneously, pure iron has good bio-compatibility and mechanical property. Compared with the polymer stents or magnesium alloy stents having a same wall thickness, the iron stents may provide a sufficient radial support for the diseased blood vessel at the diseased position. However, the corrosion rate of the pure iron is relatively slow, the iron stents of common structure design may be absorbed by the human body after a long time, during which process the diameter of the blood vessel may be restrained, so it cannot meet the requirement on the gradual increase of the blood vessel for children.
Therefore, how to design a blood vessel stent that can accelerate the absorption process of the human body and simultaneously ensure the mechanical property thereof is an urgent problem to be solved at present.